The abrasive grains obtained by the process of desintegration of an abrasive material can be divided into two groups. A first group includes monocrystal grains consisting of a single corundum crystal. The second group comprises aggregate grains containing several corundum crystals at the interfaces of which or within which there are found inclusions of ancillary impurity minerals.
The main features characterizing the abrasive material are strength and abrasiveness of the abrasive grains produced thereof as well as its cost and scarcity.
The strength of monocrystal abrasive grains is determined by the strength of the corundum crystals. These grains feature, as a rule, a higher strength as compared to the aggregate grains. The aggregate grain strength depends on the size of the corundum crystals contained in these grains, the quantity and type of the ancillary minerals such as impurities and additions found, as inclusions, at the interfaces of the corundum crystals, and on the size of these inclusions. As a rule, the smaller the size of the corundum crystals and the quantity and size of the impurity mineral inclusions and the higher the strength of these minerals, the higher the strength of the aggregate grains.
The abrasiveness of the grains is determined by the hardness of the corundum crystals, impurities, and additions and depends on the ability of the grains to self-sharpen, that is, to form new cutting faces upon destruction of the grains in the process of metal working.
All the abrasive materials are produced by melting the starting abrasive material followed by cooling of the resulting melt, in the course of which the melt is crystallized. The desired structure of the abrasive material (grain size of the main crystals, inclusion size and so on) is achieved by selecting the mode of cooling of the melt.
There are known abrasive materials consisting of corundum crystals. A typical representative of these abrasive materials is white synthetic corundum. This synthetic corundum comprises inclusions of the impurity mineral Na.sub.2 O.11Al.sub.2 O.sub.3 which are from 50 to 700 microns in size (A.P.Garshin et al., "Abrasivnye materialy", 1983, Masinostroenie (Leningrad), pp.119-123).
To obtain the white synthetic corundum, technically pure aluminum oxide, that is alumina, is used as a source material.
In the course of processing the white synthetic corundum to obtain abrasive grains, part of the impurity mineral, particularly that present in the form of coarse inclusions having size from 300 to 700 microns, is ground down to the size of slime particles and removed. As a result, the obtained abrasive grains become somewhat enriched in corundum crystals as compared to the starting abrasive material, that is, feature a higher strength. Part of the abrasive grains obtained in processing white synthetic corundum are monocrystal grains, and part, aggregate grains containing inclusions of Na.sub.2 O.11Al.sub.2 O.sub.3 sized from 50 to 250 microns.
Such abrasive materials produced by melting alumina are disadvantageous in that these contain the high-alumina sodium aluminate Na.sub.2 O.11Al.sub.2 O.sub.3 formed in the process of making the abrasive material due to the presence of sodium oxide in the alumina and crystallized out at the corundum crystal interfaces. The presence of the high-alumina sodium aluminate at the corundum crystal interfaces, which are stress concentrators, results in a lowered mechanical strength of the aggregate grains produced from this material. Moreover, since the high-alumina sodium aluminate features a lower hardness than that of corundum, the presence of said inclusions results in a decreased abrasiveness of the grains.
Also known are abrasive materials produced by melting alumina with adding to the melt titanium, chromium, vanadium oxides or a mixture thereof followed by cooling of the obtained melt (A. P.Garshin et al., "Abrazivnye materialy", 1983, Mashinostroenie (Leningrad), pp.123-126).
In the process of manufacturing these materials, any one of said additions forms solid solutions of these compounds in the aluminum oxide, whereby the strength of the corundum crystals increases, that is, the strength of the abrasive grains obtained from this material becomes higher.
However, the abrasiveness of the grains made from this material is not sufficiently high, as in the case with the previously mentioned material, due to the presence of the inclusions of Na.sub.2 O.11Al.sub.2 O.sub.3 having size from 50 to 250 microns.
Besides, each of said additions present in the abrasive material brings its own shortcomings. The abrasive material produced with adding titanium oxide contains inclusions of titanium nitrides and carbides which results in the loss of strength of the abrasive grains in manufacturing abrasive tools. This is attributed to the fact that, in the process of manufacturing abrasive tools, during thermal treatment of the grains of this material at a temperature of about 1100 .degree. C., oxidation occurs of the titanium carbides and nitrides contained in the abrasive material, accompanied by changes in volume of these, which results in the loss of strength of the abrasive grains. The abrasive material produced with adding chromium oxide contains metallic chromium inclusions tending to deteriorate the abrasive properties of the tools to be manufactured therefrom, namely, to increase the probability of burns during metal working (grinding). As for the vanadium additions, the use of these in the abrasive materials proved to be impractical due to scarcity and high cost of vanadium.
There is known an abrasive material containing crystals of corundum and baddeleyite (zirconium oxide). In this material, the corundum crystals from 10 to 70 microns in size are bonded with each other by fine-crystal corundum-baddeleyite eutectic with crystals measuring 1 to 5 microns (A.P.Garshin et al., "Abrazivnye materialy", 1983, Mashinostroenie (Leningrad), pp.126-131).
The strength of the abrasive grains produced from such an abrasive material is higher than that of the abovementioned materials, which is attributed, in the first place, to the presence of the corundum-baddeleyite eutectic.
However, since baddeleyite features a lower hardness as compared to corundum, the abrasiveness of the abrasive grains is not adequate.
Moreover, the baddeleyite present in the abrasive material makes this abrasive material suitable for manufacturing abrasive tools on an organic bond only, since at a temperature of 1100 .degree. C. (the thermal treatment temperature in manufacturing abrasive tools), the zirconium oxide undergoes modifications accompanied by considerable changes in volume, whereby a significant loss of strength of the abrasive grains, down to destruction thereof takes place.Another shortcoming of this material is its scarcity and high cost because of the zirconium oxide present therein.